Voice coil motors (VCMs) are actuators used to drive electromagnetic loads. Applications typically use VCMs due to their small size, low cost, and shock resistance. Moreover, VCM applications frequently require both forward and reverse VCM operations. For example, VCMs are used to move the read/write head in hard disk drive applications and are used to focus lenses in imaging applications.
Generally, VCMs include at least a permanent magnetic circuit and a coil. In a closed loop feedback system, a VCM interacts with a bridge circuit that drives a current through an electromagnetic load of the VCM.
To drive the current, a first set of analog circuitry that provides a dynamic, or linear, driving operation is often used in conjunction with the VCM. Dynamic drivers provide reliably linear drive signals to the electromagnetic load. However, dynamic drivers are inefficient as they dissipate a relatively large amount of power. Designers generally use dynamic drivers only in circumstances in which it is important to minimize noise and in which electromagnetic compatibility (EMC) is critical.
As an alternative to drive the current, a second set of circuitry that provides a pulse width modulation (PWM) driving operation is used in conjunction with the VCM. PWM drivers are more efficient and dissipate relatively less power than dynamic drivers. However, PWM drivers generate a great deal of radiative and conductive noise that can interfere with sensitive circuit operations proximate or connected to the VCM.
Previous VCM driving schemes generally provide systems that use only the dynamic driving operation or the PWM driving operation. In some circumstances in which both the dynamic and the PWM driving operations are used with respect to the same motor, it is often the case that the motor cannot also be operated in both a forward direction and a reverse direction for each of the dynamic and the PWM driving operations.
In some other circumstances in which both the dynamic and the PWM driving operations are performed with respect to the same motor, each driving operation is associated with a different set of core analog components in the feedback loop. Switching between the different sets of core analog components requires a transition period of time between the implementation of the different modes. Additionally, switching between the different sets of core analog components introduces an output current error between modes.
Accordingly, there exists a need for an improved voice coil motor driver that is capable of driving a load using both a PWM mode and a linear mode.